In an Ethernet ring protection system of RFC 3619, a plurality of nodes is connected to form a ring on which a plurality of Ethernet ring protection domains can be defined. In a domain it is comprised a master node, several transmit nodes and a set of protected service Virtual Local Area Networks (VLANs) for forwarding user data, wherein two ports of the master node on the ring are respectively defined as a primary port and a secondary port, and the protected service VLAN are defined as the protection service VLAN. When the link on the ring is intact, the master node blocks the forwarding function of the protection service VLAN of the secondary port and no loop is produced in the network, which avoids the “broadcast storm” caused by a loop in the network. When a fault occurs on a link in the ring, the master node unblocks the forwarding function of the protection service VLAN of the secondary port and user data can pass through the secondary port of the master node to guarantee the connectivity of services. An Ethernet ring protection control message is the communication and control message between nodes in a domain. When the node blocks the port, the Ethernet ring control message can still pass through without being affected. In the normal operation of the Ethernet ring protection system, each node on the ring records the current state of the Ethernet ring as intact (UP) or faulted (DOWN).
In the prior art, the Ethernet ring automatic protection system in the normal operation, as shown in FIG. 1, is composed of nodes S1, S2, S3 and S4. On the ring formed by these four nodes an Ethernet ring protection domain is configured, in which the master node is S2 (MASTER) and other nodes S1, S3 and S4 are transit nodes (TRANSIT). The two in-ring ports of the master node S2 are respectively a primary port and a secondary port, wherein the port 2 is the primary port (P) and the port 1 is the secondary port (S). Additionally, a protection service VLAN, in which each port on the ring is configured, are also defined in the domain. When the link on the ring is intact, the master node S2 blocks the data forwarding function of the protection service VLAN of the secondary port 1 and prevents a loop from being formed in the network, to avoid the “broadcast storm”. When the link on the ring has malfunction, the master node S2 unblocks the data forwarding function of the protection service VLAN of the secondary port 1 to make the data in the service VLAN to be reconnected again.
The master node sends a ring network link health-check message (HELLO) to the primary port periodically. When the link on the ring is intact, the Hello message passes through each transit node successively and reaches the secondary port of the master node. The Hello message can be received by the master node through the secondary port blocked. If the master node fails to receive the HELLO message in a prescribed time period, it is considered that a fault occurs in the link on the ring.
When detecting a fault occurring on an adjacent link, a transit node sends a link fault alert message (LINK-DOWN), which is transmitted through the link on the ring and reaches the master node. If the secondary port of the master node is blocked, the LINK-DOWN message also passes through the secondary port of the master node and reaches the master node. When the master node receives the LINK-DOWN message at the primary or secondary port, it is considered that a fault occurs on the link in the ring.
When detecting a fault occurring on the link in the ring, the master node records the state of the ring network as DOWN, unblocks the forwarding function of the protection service VLAN of the secondary port, updates a Media Access Control (MAC) address table of the port on the ring and sends a ring network fault notification message (FLUSH-DOWN), informing other nodes on the ring to perform a link fault switching. After receiving the FLUSH-DOWN, a transit node records the state of the ring network as DOWN, updates a MAC address table of the port on the ring, and the data in the protection service VLAN begin to be transmitted through a new path.
When detecting a fault occurring on an adjacent link, the node blocks the forwarding function of the protection service VLAN of the corresponding port. When the fault disappears, it does not unblock the forwarding function of the protection service VLAN of the port immediately and moves into a PREFORWARD state. During the LINK-DOWN period, the master node still sends the HELLO message out from the primary port. If the fault in the link disappears, the HELLO message can pass through the blocked port and the link with the fault having disappeared. If the HELLO message is received at the secondary port, it is considered that the link on the ring is restored, and the mast node records the state of the ring network as UP, blocks again the forwarding function of the protection service VLAN of the secondary port, updates the MAC address table of the port on the ring and sends a ring network restoration notification message (FLUSH-UP) to the ring, informing other nodes on the ring to perform a link restoration switching. After receiving the FLUSH-UP, a transit node records the state of the ring network as UP, updates the MAC address table of the port on the ring, unblocks the forwarding function of the protection service VLAN of the port in the PREFORWARD state, and the data in the protection service VLAN begin to be transmitted through a new path.
When configuring the Ethernet ring protection domain, the master node does not know, at the initialization stage, whether the current state of the ring network is UP or DOWN and therefore can not enter a correct state. As shown in FIG. 2, the state of the ring network is UP, the nodes S1, S3 and S4 have already been configured as transit nodes of the ring network protection domain. After being configured as the master node, the node S2 begins initializing, however, the node S2 does not know the current state of the ring network. If the node S2 sets the state of the ring network as DOWN and unblocks the secondary port 1 and the primary port 2, a loop will be formed in the whole ring network after the master node is configured and the “broadcast storm” will exist continuously.